Relates to: T25524
When generating a shadow pass, if you add 6 spot lamps -- intensity 0.001, non-shadow-casting -- at orthographic points (top, bottom, left, right, front, back) the results of the shadow pass are drastically different (and, in my opinion, *better*) than it is without the 6 lamps. See attached image for an example of how the shadow pass changes (though the main image doesn't change at all, or not enough to be discernible).
**IN THE ATTACHED BLEND*** render the current scene, which will return the shadow pass to the render window. Notice how it only shows shadows that have been cast by other surfaces (including surfaces on the same mesh, as with suzanne's ears and nose). Then turn off the renderability of the "DummyLamps" group and rerender. This is how the shadow pass is typically returned. The shadow details, especially on suzanne's face, are lost due to the "self-casting" shadow (presumably caused by the normals facing away from the only active lamp in the scene, similar to how a diffuse shader behaves).
For compositing purposes, the shadow pass with the six-lamp hack is much more useful, especially when compositing with real-world objects.
Fellow blenderer Sebastian König seems to agree with me on this (though we rely on Ton to make the decision; this is our second attempt to sway him!), and has posted a 5-minute video demonstrating the issue when compositing over real footage: http://vimeo.com/18589305
This is a followup on a less specific bug post that has since been closed: http://projects.blender.org/tracker/index.php?func=detail&aid=25524&group_id=9&atid=498
Other conversations have been held recently at blenderartists: http://blenderartists.org/forum/showthread.php?t=206440 and http://blenderartists.org/forum/showthread.php?t=206226